Depth gauges are devices that are widely used in some types of use, such as for example for diving. Several types of devices exist. Among these, capillary depth gauges are well known.
A conventional capillary depth gauge includes a glass tube closed at one end. When it is immersed in water, air remains trapped in the tube, on the closed side. Water penetrates through the aperture and compresses the air. The pressure exerted on the area of trapped air increases as a function of the increase in depth. This effect is used to display the depth reached, using a scale provided in immediate proximity to the depth gauge or directly thereon. Thus, during use in diving, when a diver moves away from the surface of the water, the air is compressed and depth can be measured as a function of the position of the air-water transition area in the tube.
It is often difficult to read this type of depth gauge since the contrast between air (transparent gas) and water (transparent liquid) is low. The refractive index variation as a function of the presence of water in the capillary tube is manifested as a variation in the reflective index. It is this inequality in the reflection coefficient that characterizes the contrast. To improve the contrast, the capillary tube can be given a particular shape. For example, the round profile can be replaced by a triangular profile. When the tube is filled with air, in some incident angle conditions, the difference in refractive index between the air and the capillary tube material produces total reflection of the light. This total light reflection is used to reflect an image of whatever is on the side of the capillary tube, such as a white background, characters or a scale. When the tube is filled with water, the difference in index between the water and capillary tube material is small or nonexistent. There is no longer total reflection of the light. Light passes through the capillary tube and the water. The capillary tube background then becomes visible. If an intensely coloured background is provided, for example a black background, the background becomes clearly visible. This type of solution corresponds to the depth gauge disclosed in GB Patent No. 919953.
With this solution, drops of water form in the capillary tube, since the difference in pressure due to the capillary effect at the air-water interface is not uniform. The depth gauge disclosed in this document thus provides limited readability. The optical effect obtained does not allow easy and precise reading.